1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a copying machine and a facsimile machine, using an electrophotographic system.
2. Description of the Related Art
Conventionally, an image forming apparatus of an electrophotographic system generally forms a toner image through processes of charging a photosensitive member, exposing, and developing and ultimately transfers the toner image to a recording material to form an image on the recording material.
FIG. 24A schematically illustrates the surrounding of a drum-type photosensitive member (photosensitive drum) included in a conventional image forming apparatus of the electrophotographic system. At the image formation, a charging device 202 substantially uniformly charges the surface of a photosensitive drum 201. The charging device 202 is connected to a high voltage power supply for charging. An exposure device 203, such as a laser scanner and an LED, exposes the surface of the charged photosensitive drum 201 based on image data. A developing unit 204 uses toner t to develop (visualize) a toner image from an electrostatic latent image formed on the photosensitive drum 201. The developing unit 204 is connected to a high voltage power supply for developing. A voltage with opposite polarity to the charged polarity of the toner at the time of development is applied to a transfer device 205 to transfer the toner image formed on the photosensitive drum 201 to a transferred body r. The transfer device 205 is connected to a high voltage power supply for transferring (not illustrated).
A color image forming apparatus repeats the processes up to the transfer for toners of, for example, four colors, yellow (Y), magenta (M), cyan (C) and black (Bk), and superimposes the color toner images. The transferred body r is a recording material, such as paper, carried by a recording material carrier in a direct transfer system. The transferred body r is an intermediate transfer member in an intermediate transfer system.
Japanese Patent Application Laid-Open No. H11-102145 discloses an image forming apparatus of an electrophotographic system that applies voltage from one high voltage power supply (common power supply) to a charging device and a developing device in order to downsize the apparatus. The image forming apparatus of Japanese Patent Application Laid-Open No. H11-102145 includes one high voltage power supply that can apply AC voltage of a rectangular wave at a predetermined duty ratio to a charging device and a developing unit that is not in contact with the photosensitive drum.
However, the configuration of applying voltage (voltage for charging and/or developing) from the common power supply to the charging device and the developing device has the following problem.
As illustrated in FIG. 24B, when voltage is applied from the common power supply to the charging device and the developing device, the voltage is also applied to the developing device at the start of initial operation of the image forming apparatus, at the same time as the application of the voltage to the charging device. The “initial operation” denotes initial operation after the control is reset, such as when the main power supply of the image forming apparatus is turned on and when there is a paper jam (jam).
Therefore, if the developing device 204 is in contact with the photosensitive drum 201, the following occurs when the application of the voltage is started. More specifically, the voltage is applied to the developing unit 204 that is in contact with a section of the surface of the photosensitive drum 201 that is not charged, in a time period in which the surface of the photosensitive drum 1 moves in a section A from a charge section Nc to a developing portion Nd after the start of the application of the voltage.
Therefore, the toner t is electrostatically pulled toward the photosensitive drum 201 in a section A′ on the photosensitive drum 201 that passes through the developing portion Nd in a time period in which the surface of the photosensitive drum 201 moves in the section A. The entire surface of the photosensitive drum 1 at the section is developed.
The amount of toner at the section A′ depends on the state of the surface potential of the photosensitive drum 201. For example, when the surface potential of the photosensitive drum 201 is substantially the same potential as the earth (around 0 V), the section A′ is developed by toner substantially at 100% density. The toner is further transferred from the surface of the photosensitive drum 201 to the transferred body r. The toner may cause a defect in the image transferred to the recording material. For example, if the transferred body r is an intermediate transfer member, the toner is adhered to an unintended part of the image. If the transferred body r is a recording material carrier, unintended toner is adhered to the back surface of the recording material or to a first surface in double-sided printing.